In satellite retransmission of communication signals, two linearly polarized signals, rotated 90 degrees from each other, are used. In less expensive installations for receiving such signals, the feed horn for the receiving system is installed with the orientation parallel to the desired signal polarization. The other polarization is not detected and is simply reflected back out of the feed horn. For more expensive installations, the entire feed horn and low noise amplifier system is mounted on a rotator similar to the type used on home television antennas to select the desired signal polarization.
While the above-mentioned systems are cost effective, they are mechanically cumbersome and limit system performance. Other prior art signal polarization rotators electrically rotate the signal field in a ferrite media. While such rotators eliminate the mechanical clumsiness of the above-described rotators, they are expensive and introduce additional signal losses (approximate 0.2 DB) into the receiving system. See, for example, such an electronic antennae rotator marketed under the trade name "Luly Polarizer" by Robert A. Luly Associates, P. O. Box 2311, San Bernardino, CA.
The present invention eliminates the mechanical disadvantages of several prior art rotators and eliminates signals losses associated with other prior art rotators. A signal detector constructed according to the principles of the present invention comprises a transmission line having a signal receiver probe portion ("RP portion") and a signal launch probe portion ("LP portion") mounted in dielectric rod at the one end of a circular waveguide and a rectangular waveguide perpendicularly coupled to the circular waveguide. The RB portion of the transmission line detects polarized incoming signals in the circular waveguide and the LP portion launches the detected signal into the rectangular waveguide for transmission to a low noise amplifier ("LNA").
In the preferred embodiment, the transmission line, by its coupling to the insulator rod, may be rotated continuously and selectively by a servo motor mounted on the waveguide assembly. As the RP portion rotates to receive the desired signal, the LP portion also rotates. However, the launched signal or the signal received at the LNA is unaffected because rotation of the LP portion is about its axis of symmetry in the rectangular waveguide. The RP portion in the circular waveguide rotates between the two orthogonally polarized signals impinging on the feed horn. By rotation to the desired polarization, that signal is received and the other reflected. The selected signal is then conducted along the transmission line to the rear wall of the circular waveguide portion of the feed horn and is launched into the rectangular waveguide by the LP portion.